Experimental measurements are described characterizing the nonlinear index change over the range from 500 to 1550 nm induced in an ytterbium (Yb3+)-doped twin-core fiber by a 980 nm pump. At 1550 nm, a phase change of \pi is induced with as little as 14 mW of pump power for a signal loss of only 0.2 dB. By allowing the doped fiber to lase and observing the associated clamping of the induced phase change, we show that a digital nonlinear response can be achieved in which a constant, pump-power-insensitive, phase change is induced for all pump powers above a certain threshold. This lasing induced clamping of the phase change also demonstrates that the nonlinear effect is population dependent as opposed to thermal. The pump-induced phase change is observed to increase for shorter signal wavelengths, which suggests that the effect is due principally to pump-induced changes in the strong ultraviolet (UV) absorptions of Yb3+. This observation is accurately predicted by a theoretical analysis that takes into account absorptions in both infrared and ultraviolet regions. This analysis shows that Yb3+ may be suitable for low-power all-optical switching applications in both 1300 and 1550 nm telecommunications windows when the speed of response is not a critical parameter.
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